DENVER -- Two motorists have filed a class-action lawsuit alleging the city illegally delegated photo-radar enforcement authority to a private contractor, and rewarded the contractor for taking more photos.

The suit, filed in Denver District Court, claims the city paid ACS State Local Solutions, formerly Lockheed Martin IMS, more than $3 million based on the number of photos taken rather than the value of the photo radar equipment, as the law requires.

Denver uses cameras in white roadside vans to catch on film speeding motorists and their license plates. Tickets are then mailed to the violators.

ACS receives about $136,000 a month for 12,099 photos, according to the suit. The suit also alleges the police aim to take as close to 12,099 images as possible without exceeding the number, allowing a photo radar technician to leave high-enforcement areas if the number of images is approaching the monthly cap.

Dean Blanken of Lakewood and Thomas Godwin of Denver filed the lawsuit Monday on behalf of more than 150,000 residents ticketed as a result of the photo radar system.

Andrew Low, lawyer for ACS State and Local Solutions, declined to comment.

The plaintiffs are seeking restitution for the fines and penalties.

The lawsuit comes after a recent ruling by Denver County Judge Mary Celeste, who said the photo-radar program illegally gave police powers to a private contractor, who prepared and sent the summonses.

The program also violates state law by appearing to compensate the contractor based on the volume of tickets issued, Celeste said.

The program was suspended last week to bring it into compliance, city officials said.

Denver police have deployed the white vans in neighborhoods to cut down on speeding and to improve neighborhood safety. They said that the system is effective and they plan to use it again.

Boulder and Fort Collins also use photo radar to catch speeders. Boulder officials said they keep the power of writing tickets in the police department.